Rfid system

ABSTRACT

An RFID system includes an antenna of a reader/writer and an antenna of an RFID tag. Transmission and reception of a high-frequency signal of a UHF band is performed between the antenna of the reader/writer and the antenna of the RFID tag that are arranged so as to be adjacent to each other. A loop antenna including a loop conductor is used as the antenna of the reader/writer, and coil antennas including a plurality of laminated coil conductors are used as the antenna of an RFID tag. In addition, the conductor width of the loop conductor in the loop antenna is greater than the conductor widths of the coil conductors in the coil antennas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an RFID (Radio Frequency Identification) system and, in particular, to a UHF band RFID system used for short-distance communication between a reader/writer and an RFID tag.

2. Description of the Related Art

As a system for managing articles, an RFID system that establishes, on the basis of a non-contact method, communication between a reader/writer and an RFID tag, and transmits information between the reader/writer and the RFID tag is known. The RFID tag includes an RFIC chip used to process a wireless signal and an antenna used to transmit and receive the wireless signal, and predetermined information is transmitted and received as a high-frequency signal between the antenna of the RFID tag and the antenna of the reader/writer through a magnetic field or a radio wave.

Since the RFID tag is to be attached to an article, a reduction in size thereof is required.

As an RFID system that uses a small RFID tag, for example, a system is disclosed in Japanese Unexamined Patent Application Publication No. 2002-183676 where an RFID tag, in which a minute antenna coil is formed on a wireless IC chip, is utilized and by moving a resonance body including a capacitor and a coil, provided in a leading end portion of a reader/writer, closer to this tag, information is read and written.

However, in the RFID system disclosed in Japanese Unexamined Patent Application Publication No. 2002-183676, when the RFID tag is mounted to a mother substrate, such as printed wiring board, for example, the RFID tag is influenced by a metallic substance, such as another mounted component or a circuit pattern, for example, provided in the printed wiring board, and a communication distance is reduced or no communication is established. In addition, due to the influence of the metallic substance, the resonance frequency of the resonance body deviates and the transmission efficiency of a high-frequency signal is reduced, in some cases. In particular, when a metallic body is located adjacent to a portion in which the RFID tag is disposed, the deviations of the resonance frequency and the reduction in transmission efficiency becomes significant.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of the present invention provide an RFID system that is capable of maintaining a communication distance and that is superior in terms of the transmission efficiency of a high-frequency signal, even if it is mounted in a mother substrate.

In an RFID system according to a preferred embodiment of the present invention, preferably, a loop antenna including a loop conductor is used as an antenna on a reader/writer side, a coil antenna including a plurality of laminated coil conductors is used as an antenna on an RFID tag side, and a conductor width of the loop conductor in the loop antenna is greater than a conductor width of the coil conductor in the coil antenna.

Since the loop antenna defined by the loop conductor is used as the reader/writer-side antenna, and the conductor width of the loop conductor in the loop antenna is greater than the conductor width of the coil conductor in the coil antenna, it is possible to concentrate a magnetic flux on the center line of a winding axis in the loop antenna with a conductor loss in the loop antenna being reduced. In addition, since the loop of the magnetic flux becomes large, it is possible to radiate the magnetic flux farther. Furthermore, since the coil antenna formed by laminating the plural coil conductors is used as the RFID tag-side antenna, and the conductor width of the coil conductors is relatively small, it is possible to reduce a stray capacitance component occurring between the coil antenna and a metallic substance in a mother substrate, and it is possible to minimize the influence of the metallic substance. Accordingly, it is possible to improve the degree of coupling between the RFID tag and the reader/writer, and it is possible to maintain a sufficient communication distance. Therefore, it is possible to provide an RFID system that is superior in terms of favorable transmission efficiency of a high-frequency signal.

According to various preferred embodiments of the present invention, it is possible to provide an RFID system that is capable of maintaining a sufficient communication distance and that is superior in terms of the transmission efficiency of a high-frequency signal.

The above and other elements, features, steps, characteristics, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an RFID system according to a preferred embodiment of the present invention.

FIGS. 2A and 2B illustrate the RFID system, wherein FIG. 2A is a plan view showing a positional relationship between a reader/writer-side antenna and an RFID tag-side antenna, and FIG. 2B is a side view.

FIG. 3 is an exploded view of a power feeding substrate in the RFID system.

FIGS. 4A and 4B illustrate the reader/writer-side antenna in the RFID system, wherein FIG. 4A is a perspective view from a back surface side, and FIG. 4B is a circuit diagram.

FIG. 5 is an explanatory diagram illustrating a usage pattern of the RFID system.

FIG. 6 is an explanatory diagram illustrating an operating principle in the RFID system.

FIG. 7 is an explanatory diagram illustrating an operating principle in the RFID system.

FIG. 8 is a plan view illustrating an example of a modification to the reader/writer-side antenna in the RFID system.

FIG. 9 is a cross-sectional view illustrating an RFID tag according to a second example of a preferred embodiment of the present invention.

FIG. 10 is an explanatory diagram illustrating a magnetic field radiation state of the RFID tag illustrated in FIG. 9.

FIG. 11 is an exploded perspective view illustrating a laminated structure of the RFID tag illustrated in FIG. 9.

FIG. 12 is a cross-sectional view illustrating an RFID tag according to a third example of a preferred embodiment of the present invention.

FIG. 13 is an explanatory diagram illustrating a magnetic field radiation state of the RFID tag illustrated in FIG. 12.

FIG. 14 is an exploded perspective view illustrating a laminated structure of the RFID tag illustrated in FIG. 12.

FIG. 15 is a cross-sectional view illustrating an RFID tag according to a fourth example of a preferred embodiment of the present invention.

FIG. 16 is an explanatory diagram illustrating a magnetic field radiation state of the RFID tag illustrated in FIG. 15.

FIG. 17 is an exploded perspective view illustrating a laminated structure of the RFID tag illustrated in FIG. 15.

FIG. 18 is a cross-sectional view illustrating an RFID tag according to a fifth example of a preferred embodiment of the present invention.

FIG. 19 is an explanatory diagram illustrating a magnetic field radiation state of the RFID tag illustrated in FIG. 18.

FIG. 20 is a cross-sectional view illustrating an RFID tag according to a sixth example of a preferred embodiment of the present invention.

FIG. 21 is an explanatory diagram illustrating a magnetic field radiation state of the RFID tag illustrated in FIG. 20.

FIG. 22A is an explanatory diagram illustrating a magnetic field radiation state in an RFID system utilizing the RFID tag according to the fifth example, and FIG. 22B is an explanatory diagram illustrating a magnetic field radiation state in an RFID system utilizing the RFID tag according to the sixth example of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, RFID systems according to preferred embodiments of the present invention will be described with reference to accompanying drawings. In addition, in each drawing, a common reference character is assigned to the same component or the same portion, and redundant description thereof is omitted.

RFID System and First Example of RFID Tag

An RFID system according to a preferred embodiment is preferably a system in which the transmission of information is performed between a reader/writer and an RFID tag on the basis of a non-contact method, and more specifically, is an RFID system in which the transmission and reception of a high-frequency signal of a UHF band or an SHF band is performed between the antenna of a reader/writer and the antenna of an RFID tag, which are disposed so as to be adjacent to each other with a distance ranging from several mm to several cm therebetween.

First, with reference to FIGS. 1 to 4B, the configurations of a reader/writer in the RFID system and an RFID tag as a first example of a preferred embodiment of the present invention will be described.

As illustrated in FIG. 1, the reader/writer preferably includes an antenna head 1 including a loop antenna 10 provided on the front surface of a supporting member 2. The loop antenna 10 is preferably defined by a loop conductor 11 of approximately one turn, whose power feeding ends are a power feeding portion 11 a and a power feeding portion 11 b, and the power feeding portion 11 a and the power feeding portion 11 b are connected to an information processing circuit in the main body of the reader/writer, not illustrated.

An RFID tag 20 preferably includes a coil antenna 30, formed by laminating a plurality of coil conductors embedded in a power feeding substrate 40, and an RFIC element 50 connected to the coil antenna 30. The coil antenna 30 preferably includes a first coil antenna 31 and a second coil antenna 32 adjacently disposed within the power feeding substrate 40 so that the winding axes of the coil antennas 31 and 32 are parallel or substantially parallel to each other, and the coil antennas 31 and 32 are magnetically coupled to each other. In addition, while being described hereinafter in detail, the coil antenna 30 preferably further includes a third coil antenna 31. The coil antenna 30 is provided within the power feeding substrate 40, and the power feeding substrate 40 is defined by a laminated body formed by laminating a plurality of dielectric layers. The power feeding substrate 40 is mounted on a mother substrate 60, such as a printed wiring board, for example, and the RFIC element 50 is mounted on the front surface of the power feeding substrate 40. The RFIC element 50 preferably includes a logic circuit, a memory circuit, and other suitable circuit elements, and is connected, as a bare IC chip or a packaged IC chip, to terminals 36 and 37 on the power feeding substrate 40 through input-output terminals on the back surface thereof.

As illustrated in FIGS. 2A and 2B, in planar view of the loop antenna 10 on a reader/writer side and the coil antenna 30 on an RFID tag 20 side, an area occupied by the loop antenna 10 is approximately equal to or slightly greater than the total area of an area occupied by the coil antenna 30, namely, an area occupied by the first coil antenna 31 and an area occupied by the second coil antenna 32. In addition, the area occupied by the loop antenna 10 is an area on an inner side with respect to a periphery of the loop antenna 10, and the area occupied by the coil antenna 30 is an area on an inner side with respect to the peripheries of the coil antennas 31 and 32.

In addition, in the present preferred embodiment, the loop conductor 11 of the loop antenna 10 is preferably wound on a plane surface, and the coil conductors of the coil antennas 31 and 32 are preferably primarily wound in a lamination direction. Furthermore, the loop antenna 10 is configured such that the conductor width of the loop conductor 11 in the loop antenna 10 is preferably greater than the conductor widths of the coil conductors in the coil antennas 31 and 32. Specifically, when the conductor width of the loop conductor 11 in the loop antenna is denoted as W1 and the conductor widths of the coil conductors of the coil antennas 31 and 32 are denoted as W2, the conductor width of the loop conductor 11 is set so as to satisfy a relationship of W1>W2.

As illustrated in FIG. 3, the power feeding substrate is preferably defined by a laminated body formed by laminating a plurality of ceramic dielectric layers 41 a to 41 f, and includes therein the coil conductors 31 a to 31 d and 32 a to 32 d and interlayer conductors 34 a to 34 e and 35 a to 35 e of the coil antennas 31 and 32. Each of the ceramic dielectric layers 41 a to 41 f is preferably made of dielectric material, such as LTCC (Low Temperature Co-fired Ceramic) material or other suitable material, whose relative permittivity ∈r is greater than or equal to about 6, for example. The coil conductors 31 a to 31 d and 32 a to 32 d and the interlayer conductors 34 a to 34 e and 35 a to 35 e are preferably made of low-melting-point metal material whose main constituent is copper or silver and whose resistivity is relatively small, for example. Particularly, the laminated body is preferably obtained by simultaneously sintering a coil conductor or an interlayer conductor and a plurality of ceramic dielectric layers.

In the dielectric layer 41 a, the terminal 36 and the terminal 37 are provided to be connected to two input-output terminals of the RFIC element 50, and the terminal 36 is connected to one end of the coil conductor 31 a, provided in the dielectric layer 41 b, through the interlayer conductor 34 a provided in the dielectric layer 41 a. The coil conductor 31 a preferably has a small-diameter loop shape in the surface of the dielectric layer 41 b, and the other end thereof is connected to one end of the coil conductor 31 b provided in the dielectric layer 41 c, through the interlayer conductor 34 b provided in the dielectric layer 41 b. The coil conductor 31 b preferably has a small-diameter loop shape in the surface of the dielectric layer 41 c, and the other end thereof is connected to one end of the coil conductor 31 c provided in the dielectric layer 41 d, through the interlayer conductor 34 c provided in the dielectric layer 41 c. The coil conductor 31 c preferably has a small-diameter loop shape in the surface of the dielectric layer 41 d, and the other end thereof is connected to one end of the coil conductor 31 d provided in the dielectric layer 41 e, through the interlayer conductor 34 d provided in the dielectric layer 41 d. The coil conductor 31 d preferably has a small-diameter loop shape in the surface of the dielectric layer 41 e, and the other end thereof is connected to one end of the coil conductor 33 a provided in the dielectric layer 41 f, through the interlayer conductor 34 e provided in the dielectric layer 41 e.

Furthermore, the coil conductor 33 a preferably has a large-diameter loop shape in the surface of the dielectric layer 41 f, and the other end thereof is connected to one end of the coil conductor 32 d provided in the dielectric layer 41 e, through the interlayer conductor 35 e provided in the dielectric layer 41 e. The coil conductor 32 d preferably has a small-diameter loop shape in the surface of the dielectric layer 41 e, and the other end thereof is connected to one end of the coil conductor 32 c provided in the dielectric layer 41 d, through the interlayer conductor 35 d provided in the dielectric layer 41 d. The coil conductor 32 c preferably has a small-diameter loop shape in the surface of the dielectric layer 41 d, and the other end thereof is connected to one end of the coil conductor 32 b provided in the dielectric layer 41 c, through the interlayer conductor 35 c provided in the dielectric layer 41 c. The coil conductor 32 b preferably has a small-diameter loop shape in the surface of the dielectric layer 41 c, and the other end thereof is connected to one end of the coil conductor 32 a provided in the dielectric layer 41 b, through the interlayer conductor 35 b provided in the dielectric layer 41 b. The coil conductor 32 a preferably has a small-diameter loop shape in the surface of the dielectric layer 41 b, and the other end thereof is connected to the terminal 37 provided in the dielectric layer 41 a, through the interlayer conductor 35 a provided in the dielectric layer 41 a.

Particularly, the first coil antenna 31 is configured using the small-diameter coil conductors 31 a to 31 d and the interlayer conductors 34 a to 34 e, and the second coil antenna 32 is configured using the small-diameter coil conductors 32 a to 32 d and the interlayer conductors 35 a to 35 e. Furthermore, in the present preferred embodiment, the third coil antenna 33 including the large-diameter coil conductor 33 a is also included. As illustrated in FIG. 3, the coil conductors 31 a to 31 d, 32 a to 32 d, and 33 a in the first coil antenna 31, the second coil antenna 32, and the third coil antenna 33 are wound so that the directions of currents flowing in the individual coil conductors are aligned in a same direction, i.e., the directions of induction magnetic fields generated by the currents flowing in the individual coil conductors are aligned in a same direction.

Preferably, the first coil antenna 31, the second coil antenna 32, and the third coil antenna 33 are adjacently disposed within the power feeding substrate 40 so that the winding axes of the individual coiled antennas are parallel or substantially parallel to one another, and the first coil antenna 31, the second coil antenna 32, and the third coil antenna 33 are magnetically coupled to one another. In addition, in planar view, an area occupied by the loop antenna 10 on a reader/writer side is preferably approximately equal to the area of a portion surrounded by the outside dimension of an antenna on an RFID tag 20 side, i.e., the outside dimensions of the first coil antenna 31, the second coil antenna 32, and the third coil antenna 33.

As illustrated in FIG. 4A, the reader/writer-side antenna is preferably configured as the loop antenna 10 provided in one main surface of the flat plate-shaped supporting member 2 including a rigid member, such as an epoxy resin, for example, and a coaxial cable 3 is connected to the other main surface of the supporting member 2. As illustrated in FIG. 4B, between the loop antenna 10 and the coaxial cable 3, a matching circuit is preferably provided and includes a capacitance element C and an inductance element L, and the power feeding portion 11 a and the power feeding portion 11 b in the loop antenna 10 are connected to an internal conductor 4 of the coaxial cable 3 and an external conductor 5 of the coaxial cable 3, respectively, through the matching circuit. The coaxial cable 3 is preferably configured as a 50Ω line, for example, and due to the matching circuit, matching between the impedance of the coaxial cable 3 and the impedance of the loop antenna 10 is achieved.

Next, with reference to FIG. 5 to FIG. 7, the usage pattern and the operating principle of the RFID system of the present preferred embodiment will be described.

As illustrated in FIG. 5, the antenna head 1 in the reader/writer preferably includes the loop antenna 10 provided in one main surface of the supporting member 2 and the matching circuit element including the capacitance element C and the inductance element L provided in the other main surface of the supporting member 2. This antenna head 1 is connected to a gripper 6 through the coaxial cable 3, and is preferably configured as a pen-shaped antenna capable of being used with the gripper 6 being gripped. The pen-shaped reader/writer-side antenna is further connected to a reader/writer main body, not illustrated, in a DC manner or through a magnetic field or an electromagnetic field.

The RFID tag 20 preferably includes the rectangular or substantially rectangular flat plate-shaped power feeding substrate 40 and the RFIC element 50 mounted thereon, and the RFIC element 50 is sealed using a sealing material 55, such as an epoxy resin, for example. This RFID tag 20 is mounted to the mother substrate 60, such as a printed wiring board, for example, through a joint material 56 including insulating material, such as resin, or conductive material, such as solder, for example.

In the present preferred embodiment, as illustrated in FIG. 6, the loop antenna 10 is used as the reader/writer-side antenna, the coil antenna 30 (31, 32) is used as the RFID tag 20-side antenna, and furthermore, the conductor width of the loop conductor 11 in the loop antenna 10 is preferably greater than the conductor widths of the coil conductors 31 a to 31 d and 32 a to 32 d in the coil antennas 31 and 32. Therefore, in a state in which the antenna head 1 is adjacent to the RFID tag 20, magnetic fields H1, H2, H3, and H4 indicated by dashed lines in FIG. 6 are generated, and through these magnetic fields, a high-frequency signal is transmitted and received between the loop antenna 10 and the coil antennas 31 and 32. Particularly, while maintaining a low conductor loss, the magnetic fields H1 and H2 generated due to the loop conductor 11 of the loop antenna 10 cause an aperture portion magnetic field to be concentrated and widely spread. In addition, as illustrated in FIG. 7, this magnetic flux interlinks with the coil conductors of the coil antennas 31 and 32.

In this manner, since, in the loop antenna 10 and the coil antennas 31 and 32, it is possible to concentrate a magnetic field in each aperture portion, even if the mother substrate 60 is metal or metal is disposed in the vicinity of the mother substrate 60, it is possible for magnetic fields to be intensively interlinked with the coil antennas 31 and 32. In addition, in the coil antennas 31 and 32, the conductor widths are narrowed and laminated structures are utilized, and thus, it is possible to focus the magnetic fields H3 and H4 primarily in a direction perpendicular or substantially perpendicular to the coil surfaces thereof. In addition to this, even if the mother substrate 60 is metal, capacitance occurring between the mother substrate 60 and the coil antennas 31 and 32 is relatively small, and has a small effect on a resonance frequency. Furthermore, since a high-frequency signal of the UHF band or a frequency band higher than the UHF band is utilized, even if the RFID tag is mounted to the mother substrate 60, the high-frequency signal is not significantly influenced by another mounted component mounted to the mother substrate 60 or metallic substances, such as various kinds of wiring patterns, for example, provided in the mother substrate 60. In addition, as for the magnetic fields H1 and H2 in the loop antenna 10, since the conductor width of the loop antenna 10 is relatively wide, the magnetic fields H1 and H2 in the loop antenna 10 widely spread primarily in a direction parallel or substantially perpendicular to the loop plane thereof, and even if the relative position of the loop antenna 10 in a planar direction with respect to the RFID tag 20 somewhat deviates, the magnetic fields H1 and H2 generated on the loop antenna 10 side easily interlink with the coil antennas 31 and 32, and an area in which reading and writing can be performed is increased.

The RFID system of the present preferred embodiment is configured such that the reader/writer-side antenna and the RFID tag-side antenna are disposed close to each other, and communication with the RFID tag 20 that is a target of reading and writing for the reader/writer is established with only the RFID tag 20. In this case, for example, the outside dimension of the coil antenna 10 is preferably less than or equal to about 1 cm long×about 1 cm wide, for example, and furthermore, it is possible to configure the coil antenna 10 in an extremely small size of less than or equal to about 0.5 cm long×about 0.5 cm wide, for example. Specifically, for example, when an operation frequency band is a UHF band of about 860 MHz to about 960 MHz, the size of the power feeding substrate 40 is about 3.2 mm long×about 1.6 mm wide, the outside dimension of the coil antenna 30 is about 2.5 mm long×about 1.2 mm wide, the outside dimension of the loop antenna 10 is about 3.0 mm long×about 4.0 mm wide, the conductor width of the loop conductor 11 is about 0.5 mm, and an output power value is about 1 W, it is possible to perform reading and writing even if a distance between the reader/writer-side antenna and the RFID tag-side antenna is about 6 mm. Further, by increasing the output power value or increasing the size of the power feeding substrate 40, and more particularly, the size of the coil antenna 30, it is possible to further increase the communication distance.

While the present invention has been described with reference to a specific preferred embodiment, the present invention is not limited to the above-described configuration.

For example, as illustrated in FIG. 8, the reader/writer-side antenna is not a loop antenna including one turn but may preferably be configured using the loop antenna 10 including a plurality of turns. In this case, a distance between the outermost diameter and the innermost diameter of the loop antenna 10 is the conductor width W1 of the loop conductor 11 in the loop antenna 10. In addition, while it is preferable that the loop antenna 10 includes a loop-shaped single-layer conductor, the loop conductor may preferably include a plurality of layers if each of the layers is thinner than the thickness of the coil antenna 30 in a lamination direction.

As described above, it is preferable that the winding directions of the first coil antenna and the second coil antenna are the same. If coil conductors defining individual coil antennas are configured so that the winding directions thereof are the same, currents in the individual coil antennas flow in the same direction, and induction magnetic fields due to the currents are also generated in the same direction. Accordingly, the currents generated in the individual coil antennas do not cancel each other out, and the energy transmission efficiency of a high-frequency signal is improved. Therefore, a communication distance between the reader/writer-side antenna and the RFID tag-side antenna is increased. In addition, the coil antennas have a laminated structure, and when the coil antennas are arranged in a position at which the winding axes of two coil antennas overlap with each other in planar view, it is possible to enlarge the sum of opening areas in the coil antennas. As a result, since magnetic flux density increases, the communication distance further increases.

In addition, since the reader/writer-side antenna is a loop antenna and the RFID tag-side antenna is a coil antenna, communication between the reader/writer and the RFID tag is performed primarily through a magnetic field. However, in this regard, if the conductor width of the loop conductor in the loop antenna is greater than the conductor width of the coil conductor in the coil antenna, and a ratio between the outside dimension of the loop antenna and the outside dimension of the coil antenna falls within a predetermined range, when the antenna head 1 and the RFID tag are disposed at an extremely short distance from each other so that the distance is less than or equal to about 2 mm, for example, capacitive coupling, in addition to the magnetic field coupling, is provided. Accordingly, even if electric power is extremely small, it is possible to perform wireless communication. Furthermore, preferably, the conductor width of the loop antenna is relatively wide, and the conductor width of the coil antenna on the RFID tag side is relatively narrow. Therefore, even if the locations of the loop antenna and the coil antenna deviate, the change of a capacitance value between the loop antenna and the coil antenna is very small, and accordingly, the change of a characteristic is very small.

When it is intended to perform communication at such a short distance, it is preferable that the area occupied by the loop antenna of the reader/writer is about 0.2 to about 6 times as large as the area occupied by the coil antenna of the RFID tag. If the area occupied by the loop antenna is less than an area about 0.2 times as large as the area occupied by the coil antenna, it is difficult to fully transmit and receive a high-frequency signal, in some cases. On the other hand, if the area occupied by the loop antenna is greater than an area about 6 times as large as the area occupied by the coil antenna, it is difficult to concentrate the magnetic flux of the loop antenna and even if the loop antenna is arranged adjacent to the coil antenna, a region in which it is difficult to perform reading and writing, namely, a null point, tends to occur. In addition, when the loop antenna is disposed at an extremely small distance, it is difficult for capacitive coupling to occur.

In addition, it is preferable that the coil antenna is configured so that the imaginary portion of the impedance of the RFIC element and the imaginary portion of the impedance of the coil antenna have a conjugate relationship with each other at the operation frequency. Namely, it is preferable that the coil antenna provided in the power feeding substrate has a function to match the impedance of the RFIC element in addition to having a function as an antenna. While the coil antenna has a resonance frequency due to an inductance component of the coil itself and a capacitance component produced between lines, it is preferable that this resonance frequency is located near the operation frequency. It is further preferable that the real portions of the impedances coincide or substantially coincide with each other. In particular, when an antenna in which the first coil antenna and the second coil antenna are magnetically coupled to each other is used as the coil antenna, the operation frequency band may have a wider bandwidth.

In addition, the RFID tag may preferably be attached to the mother substrate using a bonding material, such as a double-stick tape, an adhesive material, or other suitable material, for example, and in this case, after being processed into a seal, a label, a tape, or other item, the RFID tag may also preferably be attached to the mother substrate. At this time, in the RFID tag, any one of the RFIC element side and the power feeding substrate side thereof may be used as the surface to be attached to the mother substrate. In particular, if the RFIC element is covered by a sealing material, it is possible to protect the RFIC element, and it is possible to attach the RFIC element to the mother substrate using the upper surface of the sealing material.

While a base material defining the power feeding substrate may also preferably be made of a typical resin material having relative permittivity ∈r in a range of about 3 to about 4, if the power feeding substrate is configured using a material such as a ceramic dielectric, for example, whose relative permittivity ∈r is high, it is possible to achieve stable operation of the RFID system. Specifically, since line-line capacitance between the coil conductors is dependent upon the quality of material provided the coil conductors, the influence of the relative permittivity of the material used for the mother substrate is reduced, and fluctuations of stray capacitance is less likely to occur. In addition, the change of the inductance value of the coil conductor is also relatively small. Therefore, the change of the resonance frequency is small, and the communication distance is ensured, regardless of a usage environment.

While the RFID tag may preferably be mounted to various mother substrates, such as a printed wiring board, for example, the RFID tag may also be mounted on a metallic plate. In this case, in the RFID tag, preferably, a surface on a side on which the RFIC element is provided is used as a mounting surface for a metallic plate, and a power feeding substrate side is used as a top surface side. Accordingly, communication with the reader/writer is more reliable. When the surface on the power feeding substrate side is used as the mounting surface for a metallic plate, the coil antenna is disposed as close to the upper side of the power feeding substrate as possible. Accordingly, it is possible to secure a path through which a magnetic flux passes, between the coil antenna and the metallic plate, and it is possible to stabilize an operation on the metallic plate.

It is preferable that the antenna head in the reader/writer is configured so that the coaxial cable extends in a direction oblique to the loop plane of the loop antenna and the coil plane of the coil antenna. By being configured in this manner, it is possible to reduce mutual interference between both of the magnetic field generated in the loop antenna and the magnetic field generated in the coil antenna and the coaxial cable.

In addition, it is only necessary for the coil antenna that is the RFID tag-side antenna to be a lamination-type coil antenna including a plurality of laminated coil conductors in a direction perpendicular to the loop plane of the loop antenna that is the reader/writer-side antenna, and the coil antenna may also be a single coil antenna.

Second Example of RFID Tag

As illustrated in FIG. 9, an RFID tag 101A according to a second example of a preferred embodiment of the present invention preferably includes an RFIC element 110 arranged to process a transmission/reception signal of a predetermined frequency, a power feeding substrate 120, and a coil antenna 130 embedded in the power feeding substrate 120.

Preferably, the RFIC element 110 is configured in a chip form, includes a clock circuit, a logic circuit, a memory circuit, and other suitable circuits, and stores necessary information, and a pair of input-output terminal electrodes not illustrated are provided in the back surface thereof. In addition, the RFIC element 110 is mounted on the power feeding substrate 120. The power feeding substrate 120 preferably includes a plurality of laminated layers whose main constituent is dielectric or magnetic material, for example.

As will be described hereinafter with reference to FIG. 11, the coil antenna 130 is preferably wound in a coil shape by laminating and connecting conductor patterns 133 a to 133 c for a coil, formed on sheets 121 c to 121 e of dielectric or magnetic material, to each other using via hole conductors 134 a. Ends of the coil antenna 130 are preferably electrically connected to the input-output terminal electrodes of the RFIC element 110 through solder bumps 115, for example.

As illustrated in FIG. 9, the RFID tag 101A is preferably attached to a base material 140, such as a printed wiring substrate or other suitable material, through an adhesive layer 141. The coil antenna 130 is preferably embedded in the power feeding substrate 120 so that the center plane A of the antenna 130 in the lamination direction is located on a side opposite to the base material 140 with respect to the center plane B of the power feeding substrate 120. Specifically, the coil antenna 130 is preferably located a distance C away from the front surface of the base material 140.

The RFID tag 101A is capable of communicating with a reader/writer in an RFID system, not illustrated, such that the RFID tag 101A and the reader/writer define an information processing system. In this information processing system, by arranging the antenna of the reader/writer adjacent to the RFID tag 101A, a magnetic flux based on a signal of a predetermined frequency (for example, a UHF band or an HF band) radiated from the antenna penetrates the coil antenna 130, and thus a current flows in the antenna 130. This current is supplied to the RFIC element 110, thereby causing the RFIC element 110 to operate. On the other hand, a response signal from the RFIC element 110 is radiated, as a magnetic field, from the coil antenna 130, and read by the reader/writer.

A magnetic field H radiated from the coil antenna 130 is indicated by dotted lines in FIG. 10. Since the coil antenna 130 is embedded in the power feeding substrate 120, and the center plane A of the antenna 130 in the lamination direction is located on a side opposite to the base material 140, such as a printed wiring substrate or other suite B of the power feeding substrate 120 in the lamination direction, the magnetic field H is primarily generated in a direction towards the antenna of the reader/writer and away from the base material 140. Therefore, an influence of a metallic substance, such as another mounted component, a conductor pattern, or other substance, for example, provided in the base material 140, is minimized, and the communication distance is not decreased.

In addition, while communication between the reader/writer and the RFID tag 101A is established primarily by a magnetic field, since the attenuation of the magnetic field with respect to a distance is greater than that of an electric field, communication is established in a relatively close state. Therefore, it is possible to establish communication with only the RFID tag to be a target to be read for the reader/writer, and there is little possibility that communication is erroneously established with a neighboring RFID tag that is not a target to be read.

It is preferable that the imaginary portion of the impedance of the RFIC element 110 and the imaginary portion of the impedance of the coil antenna 130 have a conjugate relationship with each other at the frequency of a signal used for communication. Particularly, it is preferable that the resonance frequency of the coil antenna 130 is located near the operation frequency. Further, it is preferable that the real portions of the impedances coincide or substantially coincide with each other.

In particular, when the coil antenna 130 is a lamination type coil antenna and has a relatively large aperture portion, it is possible to obtain a large inductance value with the size thereof being relatively small such that the overall size of the RFID tag 101A can be reduced. By setting the operation frequency to a short wavelength in the vicinity of about 950 MHz, for example, the size of the RFID tag 101A may be further reduced. When the frequency of a UHF band is used for communication, the RFID tag 101A may preferably have a size that is about 3.2 mm long, about 01.6 mm wide, and about 0.5 mm tall, for example.

Here, an example of the laminated structure of the power feeding substrate 120 (coil antenna 130) will be described with reference to FIG. 11. The power feeding substrate 120 is preferably obtained by forming and laminating electrodes, conductors, and via hole conductors in a plurality of sheets 121 a to 121 e whose main constituent is a dielectric material or a magnetic material, for example, and furthermore, sheet groups 121 f and 121 g used to obtain the height of the center plane A are laminated therein.

Preferably, electrodes 131 a and 131 b to be connected to input-output terminal electrodes of the RFIC element 110, not illustrated, and mounting electrodes 131 c and 131 d (to be connected to mounting terminal electrodes of the RFIC element 110, not illustrated) are provided in the sheet 121 a of the first layer, connecting conductors 132 a and 132 b are provided in the sheet 121 b of the second layer, and the conductor patterns 133 a, 133 b, and 133 c for a coil are provided in the sheets 121 c to 121 e of the third layer to the fifth layer.

The conductor patterns 133 a, 133 b, and 133 c for a coil are connected in a coil shape through via hole conductors 134 a, thereby forming the antenna 130. One end of the conductor pattern 133 a is connected to the electrode 131 a through a via hole conductor 134 b, the connecting conductor 132 a, and a via hole conductor 134 c. In addition, one end of the conductor pattern 133 c is connected to the electrode 131 b through a via hole conductor 134 d, the connecting conductor 132 b, and a via hole conductor 134 e.

When a lamination type coil antenna 130 is provided, it is possible to achieve stable operation in addition to an enlarged aperture portion. Particularly, since capacitance between the conductor patterns 133 a, 133 b, and 133 c for a coil is dependent on the quality of material therebetween (the quality of the material of the sheet), the influence of the electric permittivity of the attachment target article of the RFID tag 101A is reduced (the fluctuation of stray capacitance is less likely to occur), and a change of the inductance value of the coil is minimized. Therefore, a change of the resonance frequency is minimized, and the communication distance ensured. In particular, by using a material having a high electric permittivity for the power feeding substrate 120, the impedance of the coil within the power feeding substrate 120 is accurately determined, and becomes insusceptible to a usage environment.

Third Example of RFID Tag

As illustrated in FIG. 12, an RFID tag 101B according to a third example of a preferred embodiment of the present invention is preferably obtained by providing conductor patterns 133 a, 133 b, and 133 c for a coil in the upper portion of the power feeding substrate 120 to define the coil antenna 130, mounting the RFIC element 110 on the back surface side of the power feeding substrate 120, and providing a sealing layer 125 so as to cover the RFIC element 110. The upper surface of the sealing layer 125 is attached to the base material 140, such as a printed wiring substrate or other suitable material, for example, through the adhesive layer 141.

In the RFID tag 101B, the coil antenna 130 is preferably embedded in the power feeding substrate 120 so that the center plane A of the antenna 130 in the lamination direction is located on a side opposite to the base material 140 with respect to the center plane B of the power feeding substrate 120. Furthermore, the sealing layer 125 is disposed between the power feeding substrate 120 and the base material 140, and thus, the distance C between the coil antenna 130 and the front surface of the base material 140 greater than in the second example.

The operation of the RFID tag 101B is the same or substantially the same as the second example and in particular, since the distance C is increased, as illustrated by dotted lines in FIG. 13, the magnetic field H generated in the antenna 130 is farther away from the front surface of the base material 140 and is closer to the antenna of the reader/writer, not illustrated. Therefore, it is possible to more effectively eliminate the influence of a metallic substance such as another mounted component, a conductor pattern, or other metallic substance, for example, provided in the base material 140. In addition, by covering the RFIC element 110 using the sealing layer 125, the RFIC element 110 is protected from the external environment. Particularly, the RFIC element 110 is protected from an external mechanical shock. In addition, it is possible to prevent a short circuit caused by moisture or other contaminants, for example, from occurring.

The laminated structure of the power feeding substrate 120 (coil antenna 30) in the present third example is illustrated in FIG. 14, the sheet groups 121 f and 121 g illustrated in FIG. 11 are arranged between the sheet 121 b and the sheet 121 c, and the conductor pattern 133 c for a coil is provided in the back surface of the sheet 121 e. In addition, when the RFID tag 101B is attached to the base material 140, the sheet 121 e is preferably an uppermost layer.

Fourth Example of RFID Tag

As illustrated in FIG. 15, an RFID tag 201A according to a fourth example of a preferred embodiment of the present invention preferably includes the RFIC element 110 arranged to process a transmission/reception signal of a predetermined frequency, a power feeding substrate 220, and a coil antenna 230 embedded in the power feeding substrate 220.

As will be described hereinafter with reference to FIG. 17, the coil antenna 230 is preferably wound in a coil shape by laminating and connecting conductor patterns 233 a to 233 c for a coil, provided on sheets 221 c to 221 e of dielectric or magnetic material, to each other using via hole conductors 234 a. Ends of the coil antenna 230 are electrically connected to the input-output terminal electrodes of the RFIC element 110 through solder bumps 215, for example, respectively.

As illustrated in FIG. 15, the RFID tag 201A is preferably attached to a base material 240, such as a printed wiring substrate or other suitable material, for example, with an adhesive layer 241. As illustrated in FIG. 16, the coil antenna 230 is arranged so that the spread of a magnetic field H radiated from the antenna 230 varies depending on the top surface side and the bottom surface side of the power feeding substrate 220. Specifically, the coil antenna 230 is preferably arranged so that the opening sizes (meaning the internal diameters of individual patterns in the present application) of the conductor patterns 233 a, 233 b, and 233 c for a coil substantially increase in a direction from the bottom surface side of the power feeding substrate 220 to the top surface side thereof.

The RFID tag 201A is capable of communicating with a reader/writer in an RFID system, not illustrated, and the RFID tag 201A and the reader/writing define an information processing system. In this information processing system, by arranging the antenna of the reader/writer adjacent to the RFID tag 201A, a magnetic flux based on a signal of a predetermined frequency (for example, a UHF band or an HF band) radiated from the antenna penetrates the coil antenna 230, and thus, a current flows in the antenna 230. This current is supplied to the RFIC element 110, thereby causing the RFIC element 110 to operate. On the other hand, a response signal from the RFIC element 110 is radiated, as a magnetic field, from the coil antenna 230, and read by the reader/writer.

A magnetic field H radiated from the coil antenna 230 is indicated by dotted lines in FIG. 16. Since this coil antenna 230 is embedded in the power feeding substrate 220, and the magnetic field H generated in the coil antenna 230 spreads beyond the RFID tag 201A. Accordingly, the degree of freedom of a positional relationship with the antenna of the reader/writer, not illustrated, is increased, and it is possible to stably establish communication over a wide range.

In addition, while communication between the reader/writer and the RFID tag 201A is established primarily by a magnetic field, since the attenuation of the magnetic field with respect to a distance is greater than that of an electric field, communication is established in a relatively close state. Therefore, it is possible to establish communication with only the RFID tag to be a target to be read for the reader/writer, and there is little possibility that communication is erroneously established with a neighboring RFID tag that is not a target to be read.

It is preferable that the imaginary portion of the impedance of the RFIC element 110 and the imaginary portion of the impedance of the coil antenna 230 have a conjugate relationship with each other at the frequency of a signal used for communication. Particularly, it is preferable that the resonance frequency of the coil antenna 230 is located near the operation frequency. It is further preferable that the real portions of the impedances coincide or substantially coincide with each other.

In particular, when the coil antenna 230 is a lamination type coil antenna and has a relatively large aperture portion, it is possible to obtain a large inductance value with a relatively small coil antenna, and furthermore, the overall size of the RFID tag 201A can be reduced. By setting the operation frequency to a short wavelength in the vicinity of about 950 MHz, the size of the RFID tag 201A can be further reduced. When the frequency of a UHF band is used for communication, the RFID tag 201A may preferably have a size of about 3.2 mm long, about 1.6 mm wide, and about 0.5 mm tall, for example.

Here, an example of the laminated structure of the power feeding substrate 220 (coil antenna 230) will be described with reference to FIG. 17. This power feeding substrate 220 is obtained by providing and laminating electrodes, conductors, and via hole conductors in a plurality of sheets 221 a to 221 e whose main constituent is a dielectric material or a magnetic material, for example.

Preferably, electrodes 231 a and 231 b, to be connected to input-output terminal electrodes of the RFIC element 110, not illustrated, and mounting electrodes 231 c and 231 d (to be connected to mounting terminal electrodes of the RFIC element 110, not illustrated) are provided in the sheet 221 a of the first layer, connecting conductors 232 a and 232 b are provided in the sheet 221 b of the second layer, and conductor patterns 233 a, 233 b, and 233 c for a coil are provided in the sheets 221 c to 221 e of the third layer to the fifth layer.

The conductor patterns 233 a, 233 b, and 233 c for a coil are connected in a coil shape through via hole conductors 234 a, thereby defining the antenna 230. One end of the conductor pattern 233 a is connected to the electrode 231 a through a via hole conductor 234 b, the connecting conductor 232 a, and a via hole conductor 234 c. In addition, one end of the conductor pattern 233 c is connected to the electrode 231 b through a via hole conductor 234 d, the connecting conductor 232 b, and a via hole conductor 234 e.

When a lamination type antenna coil is used for the coil antenna 230, it is possible to achieve stable operation in addition to an enlarged aperture portion. Particularly, since capacitance between the conductor patterns 233 a, 233 b, and 233 c for a coil is dependent on the quality of material therebetween (the quality of the material of the sheet), the influence of the electric permittivity of the attachment target article of the RFID tag 201A is reduced (the fluctuation of stray capacitance is less likely to occur), and the change of the inductance value of the coil is minimized. Therefore, a change of the resonance frequency is minimized, and the communication distance is ensured. In particular, by using material having a high electric permittivity for the power feeding substrate 220, the impedance of the coil within the power feeding substrate 220 is effectively determined, and is not significantly influenced by a usage environment.

In addition, the conductor patterns 233 a, 233 b, and 233 c for a coil in each coil antenna 230 may preferably be configured using a greater number of conductor patterns. In addition, it is only necessary for the opening size of each pattern to be configured so as to substantially increase in a direction from the bottom surface side of the power feeding substrate 220 to the top surface side thereof. It is not necessary for the term “substantially” to mean that the opening size continuously increases in a step-by-step manner, and a conductor pattern for a coil, located midway, may also have the same opening size as those of patterns located above and below the conductor pattern or alternatively, the conductor pattern for a coil may also have an opening size larger than that of a pattern located below the conductor pattern. In addition, in a preferred embodiment of the present invention described below, it is also not necessary for the opening size of each pattern to continuously increases in a step-by-step manner in a vertical direction.

Fifth Example of RFID Tag

As illustrated in FIG. 18, an RFID tag 201B according to a fifth example of a preferred embodiment of the present invention is preferably obtained by mounting the RFIC element 110 on the back surface side of the power feeding substrate 220 in which the coil antenna 230 is embedded and providing a sealing layer 225 arranged to cover the RFIC element 110. The upper surface of the sealing layer 225 is attached to the base material 240 such as a printed wiring substrate or other suitable material, for example, by the adhesive layer 241.

The remaining configuration of the fifth example is the same or substantially the same as the fourth example. Accordingly, the functional effect of the fifth example is the same or substantially the same as the fourth example. In particular, by covering the RFIC element 110 using the sealing layer 225, the RFIC element 110 is protected from an external environment. Particularly, the RFIC element 110 is protected from an external mechanical shock. In addition, it is possible to prevent a short circuit caused by moisture or other contaminants, for example, from occurring. Furthermore, the sealing layer 225 is preferably disposed between the power feeding substrate 220 and the base material 240. Therefore, the distance C between the coil antenna 230 and the front surface of the base material 240 is greater than in the fourth example, and the magnetic field H radiated from the coil antenna 230 is arranged so as to be farther away from the base material 240 (refer to FIG. 19). Therefore, it is possible to reduce the influence on the magnetic field H of a metallic substance, such as another mounted component, a wiring pattern, or other metallic substance, for example, provided in the base material 240.

Sixth Example of RFID Tag

As illustrated in FIG. 20, an RFID tag 201C according to a sixth example of a preferred embodiment of the present invention is preferably obtained by providing the conductor patterns 233 a, 233 b, and 233 c for a coil, which define the coil antenna 230, so that the opening sizes thereof substantially decrease in a direction from the bottom surface side of the power feeding substrate 220 to the top surface side thereof. In addition, in the same or substantially the same manner as the fourth example, the RFIC element 110 is mounted to the bottom surface of the power feeding substrate 220 and coated by the sealing layer 225.

The remaining configuration of the sixth example is the same or substantially the same as the fourth example. In the sixth example, since the magnetic field H is radiated from the coil antenna 230 as indicated by dotted lines in FIG. 21, and the opening sizes of the conductor patterns 233 a, 233 b, and 233 c are configured so as to decrease in a direction to the top surface of the power feeding substrate 220, the magnetic field H faces the inward side of the RFID tag 201C, and a region whose magnetic flux density is relatively large is provided in a central portion of the RFID tag 201C, thereby improving a communication characteristic. In addition, the other functional effects of the sixth example are preferably the same or substantially the same as those of the fourth and fifth examples.

Magnetic Field Radiation State in Fifth Example and Sixth Example

In the RFID system according to preferred embodiments of the present invention, the loop antenna 10 is preferably used as a reader/writer-side antenna, and as an RFID tag-side antenna, for example, the coil antenna 230 is preferably used that is configured so that most of a magnetic field is generated on the reader/writer side. Accordingly, in a state in which the antenna head 1 is arranged to be adjacent to the RFID tags 201B and 201C, as illustrated in FIGS. 22A and 22B, the magnetic field H1 generated from the loop antenna 10 and the magnetic field H2 generated from the coil antenna 230 interlink with each other, a high-frequency signal is mutually transmitted between the loop antenna 10 and the coil antenna 230.

The magnetic field H1 generated from the loop antenna is preferably concentrated in the aperture portion of the loop antenna 10, and widely spread. On the other hand, in the coil antenna 230 in each of the RFID tags 201B and 201C, since most of the magnetic field H2 is generated on the reader/writer side, the magnetic field H2 is preferably concentrated in the aperture portion of the loop antenna 10. Accordingly, even if a metal material is included in or adjacent to the base material 240 or alternatively, the base material 240 is a metal material, communication performance is not deteriorated. Furthermore, by utilizing a high-frequency signal of the UHF band or a frequency band higher than the UHF band, even if the base material 240, on or in which each of the RFID tags 201B and 201C is mounted, is a metal material, the communication performance is not significantly influenced by another mounted component mounted to the base material 240 or metal members, such as various kinds of wiring patterns, for example. In addition, even if the RFID tags 101A, 101B, 201A, 201B, and 201C are used, since, from the coil antennas 130 and 230, most of the magnetic fields are generated on the loop antenna 10 side of the reader/writer, the above-described advantageous effects are achieved.

In addition, when the conductor width of the loop antenna 10 is increased, the magnetic field H1 generated from the loop antenna 10 spreads widely primarily in a direction parallel or substantially parallel to the loop plane thereof, and even if the relative position of the loop antenna 10 with respect to the RFID tags 201B and 201C slightly deviates, the magnetic fields H1 and H2 reliably interlink with each other. Therefore, it is possible to ensure necessary communication performance. Since the magnetic field H2 generated in the coil antenna 230 spreads outward when viewed from the RFID tags 201B and 201C, the degree of freedom of a positional relationship with the loop antenna 10 is significantly increased. In addition, in FIGS. 22A and 22B, the magnetic fields H1 and H2 do not graphically illustrate all of the generated magnetic fields.

In the RFID system according to preferred embodiments of the present invention, the antenna of the reader/writer and the RFID tag are preferably used in a state of being adjacent to each other, and it is possible to establish communication with only a target RFID tag. In this case, it is preferable to configure the coil antenna 230 with an outside dimension thereof being less than or equal to an outside dimension of about 10 mm long and about 10 mm wide and more preferably with a small size of less than or equal to a size of about 5 mm long and 5 about mm wide. Specifically, when it is assumed that an operation frequency band is a UHF band of about 860 MHz to about 960 MHz, the size of each of the RFID tags 201B and 201C is preferably about 3.2 mm long×about 1.6 mm wide, the outside dimension of the coil antenna 230 is preferably about 3.0 mm long×about 4.0 mm wide, the conductor width of the loop antenna 10 is preferably about 0.5 mm, and an output power value preferably is about 1 W, for example, it is possible to establish communication even if a distance between the loop antenna 10 and each of the RFID tags 201B and 201C is about 6 mm. By increasing the output power value or enlarging the size of the coil antenna 230, it is possible to further increase the communication distance.

As described above, preferred embodiments of the present invention are useful for an RFID system in which an RFID tag and a reader/writer establish communication with each other with a distance therebetween of several mm to several cm and in particular, preferred embodiments of the present invention are capable of maintaining a communication distance and superior transmission efficiency of a high-frequency signal.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

1. An RFID system comprising: a reader/writer including an antenna; and an RFID tag including an antenna; wherein transmission and reception of a high-frequency signal is performed between the antenna of the reader/writer and the antenna of the RFID tag; the antenna of the reader/writer is defined by a loop antenna including a loop conductor; the antenna of the RFID tag is defined by a coil antenna including a plurality of laminated coil conductors; and a conductor width of the loop conductor in the loop antenna is greater than a conductor width of the coil conductor in the coil antenna.
 2. The RFID system according to claim 1, wherein the coil antenna is provided in a power feeding substrate including a plurality of laminated dielectric layers.
 3. The RFID system according to claim 1, wherein the coil antenna includes a first coil antenna and a second coil antenna; and the first coil antenna and the second coil antenna are magnetically coupled to each other.
 4. The RFID system according to claim 1, wherein an outside dimension of the coil antenna is less than or equal to about 1 cm×about 1 cm in planar view.
 5. The RFID system according to claim 4, wherein an area occupied by the loop antenna is about 0.2 to about 6 times as large as an area occupied by the coil antenna, in planar view.
 6. The RFID system according to claim 2, wherein the power feeding substrate includes a ceramic laminated body; and each of the plurality of laminated dielectric layers is a ceramic dielectric layer.
 7. The RFID system according to claim 1, wherein the high-frequency signal is a signal of a UHF band or an SHF band. 